1. Field of the Invention
The invention relates to an apparatus and method for detecting an abnormal air-fuel ratio variation among cylinders of a multi-cylinder internal combustion engine.
2. Description of Related Art
Generally, in an internal combustion engine provided with an exhaust gas control system that uses a catalyst, it is necessary to control the mixture ratio of an air-fuel mixture burned in the internal combustion engine, that is, the air-fuel ratio, in order to purify toxic substances in exhaust gas using the catalyst with high efficiency. To control the air-fuel ratio, an air-fuel ratio sensor is provided in an exhaust passage of the internal combustion engine, and then feedback control is executed such that the air-fuel ratio detected by the air-fuel ratio sensor follows a predetermined target air-fuel ratio.
On the other hand, in a multi-cylinder internal combustion engine, air-fuel ratio control is usually executed over all the cylinders using the same control amount, so an actual air-fuel ratio may vary among the cylinders even when air-fuel ratio control is executed. At this time, if the variation is small, the variation may be absorbed by air-fuel ratio feedback control, and, in addition, toxic substances in exhaust gas may be purified by the catalyst. Therefore, such small variation does not influence exhaust emissions, and is not particularly problematic.
However, for example, if a fuel injection system or valve gear of an intake valve of part of the cylinders fails and, therefore, the air-fuel ratio significantly varies among the cylinders, exhaust emissions deteriorate and may be problematic. It is desirable that such a large air-fuel ratio variation that deteriorates exhaust emissions is detected as an abnormal variation.
For example, in an internal combustion engine described in Japanese Patent Application Publication No. 2010-112244 (JP 2010-112244 A), first, it is determined whether there is an imbalance in air-fuel ratio among the cylinders of the internal combustion engine on the basis of a value computed through air-fuel ratio feedback control. In the above internal combustion engine, main air-fuel ratio feedback control is executed on the basis of the result detected by an A/F sensor provided on the upstream side of a purification catalyst in an exhaust passage, and sub-air-fuel ratio feedback control is executed on the basis of the result detected by an O4 provided on the downstream side of the purification catalyst. When the average of values computed through the sub-air-fuel ratio feedback control exceeds a normal value, it is determined that there is an imbalance in air-fuel ratio among the cylinders. Furthermore, in the internal combustion engine described in JP 2010-112244 A, when it is determined that there is an abnormal air-fuel ratio variation among the cylinders in this way, the process of reducing a fuel injection duration of each cylinder in a predetermined period of time is executed, and then the cylinder in which misfire occurs because of the reduction in the fuel injection duration is identified as the cylinder in which there is an imbalance in air-fuel ratio.
In the multi-cylinder internal combustion engine, fuel injection amount changing control that forcibly changes a fuel injection amount supplied to each cylinder is executed to thereby make it possible to detect an abnormal air-fuel ratio variation among the cylinders, that is, a situation in which there is an imbalance in air-fuel ratio among the cylinders. However, even when such fuel injection amount changing control is being executed, it may be understood that air-fuel ratio feedback control that causes the air-fuel ratio to follow a predetermined target air-fuel ratio, such as a stoichiometric air-fuel ratio, is executed. In this case, as a result of executing air-fuel ratio feedback control, the fuel injection amount changed through fuel injection amount changing control may be shifted or the fuel injection amount of each cylinder that is not subjected to fuel injection amount changing control may not be kept at a desired fuel injection amount. Thus, there may be the case where an abnormal air-fuel ratio variation among the cylinders is not easily detected because of such fuel injection amount changing control. On the other hand, when air-fuel ratio feedback control is merely stopped at the time detecting such an abnormal air-fuel ratio variation among the cylinders, the air-fuel ratio deteriorates and, as a result, an abnormal air-fuel ratio variation among the cylinders may not be appropriately detected or there may occur inconvenience, such as deterioration of drivability.